The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. As used herein, the term is meant to encompass a device which utilizes the principle of amplification of electromagnetic waves by stimulated emission of radiation to produce coherent radiation in the infrared, visible or ultraviolet region. Such radiation has been used in external medical applications, such as for cauterizing, for attaching detached retinas and for removing various skin cancers.
Likewise, optical fibers have been used in a variety of medical applications. An optical fiber is a clad plastic or glass tube wherein the cladding is of a lower index of refraction than the core of the tube. When a plurality of such tubes are combined, a fiber optic bundle is produced. Optical fibers are flexible and are therefore capable of guiding light in a curved path defined by the placement of the fiber.
Fiber optic scopes have been developed for medical technology in order to enable illuminating and viewing access by the medical practitioner to the various interior parts of the body. In many medical applications, fiber optic devices have been combined with laser techniques to properly focus and apply laser radiation to interior parts of the body.
More recently, laser catheters have been constructed in which flexible or rigid hollow tubular devices (catheters) containing optical fibers are inserted into veins or arteries to illuminate internal parts of the body for diagnostic and surgical purposes. Such an application, in which fiber optic bundles are contained within a flexible catheter conduit, is described in U.S. Pat. No. 4,207,874 issued to D. S. J. Choy on June 17, 1980. This fiber optic catheter contains a combination of: (1) a fiber optic viewing bundle; (2) a light source bundle for illuminating on to be viewed; (3) a laser bundle for delivering laser light to the site for removal of tissue; (4) an annular space around the bundles for fluid supply or suction; and (5) a proximal supply and a transparent reservoir connected to the annular space. All of the above items together constitute a "laser tunneling device". The sole described use for the device is the removal of thrombus in veins for applications in the circulatory system.
The Choy device relies on visualizing the thrombus obstruction in a a vein, via the viewing bundle. It is therefore necessary to purge the blood. As no means of blocking the blood flow is shown in Choy, the Choy device can be used only when the vein is already totally obstructed. As soon as the obstruction is opened even a small amount, blood in the transparent reservoir indicates the end point of the procedure. A partial blockage which causes inadequate flow cannot be visualized or treated by the device. In the case of coronary arteries, an important treatment area for the present invention, complete blockage would cause death of the distal tissue, and so restoring blood flow at such a late state of disease would provide little clinical benefit.
M. Bass, in U.S. Pat. Nos. 3,858,577 and 4,146,019, describes a device which uses a transparent window to protect an optical fiber carrying laser radiation into a body cavity (e.g. the stomach). The window has a possible protective function, that of preventing spattering of debris from the laser tissue interaction back onto the optical fiber in the gas-purged environment. The cleanable or replacable window is in all examples recessed into a metallic or non-optically transparent holder. The design is such as to to avoid contact between tissue and the window. In addition, the cavity formed by the recess would tend to trap fluid, such as blood, absorbing the laser radiation and hindering it from reaching the target tissue.
In Bass, multiple fibers within the catheter body are described, but only for the purpose of replacement in case of fiber failure.
The Bass instrument also includes a flexible fiber optic endoscope for viewing the body cavity as an integral part of the device. Being a visual device, the information which can be provided for diagnosis by the endoscope is limited to what can be seen. In addition, the endoscope is not contained within the windowed enclosure, so the field of view in front of the endoscope must be completely purged of all non-transparent fluids, such as blood or blood substitutes. Such a purge deprives distal tissues of blood and oxygen. Therefore, the Bass instrument is clearly an instrument not intended for use, and cannot be used, in the vascular system.
J. H. Hett in U.S. Pat. No. 4,072,147, describes a device for viewing and for carrying therapeutic laser radiation into a body cavity. This endoscopic device contains a fiber optic bundle image transmitter connected to an eyepiece for viewing; a spotter light path which indicates where the endoscope is aimed; and optical fibers to deliver therapeutic radiation (which need not be visible light) to that visualized spot. This instrument further may contain a protective transparent cover over the distal end of the instrument. It also may incorporate a manually adjustable variable filter in the viewing path so as to protect physicians eye. A servo system connected to the manually adjusted filter can adjust therapeutic laser power.
The Hett instrument is designed only for direct visualization by eye and requires an optical image transmitter coherent fiber bundle. Since it is a visual device, the information about the tissue diagnosis is limited to what can be seen. Also, because visualization is used, the path from the distal end of the instrument to the tissue must be clear, but no means of purging non-transparent fluids (such as blood) is provided. The spotter beam, and hence the therapeutic radiation, is delivered to a single location in front on to the side of the distal end of the device: "The image (therapeutic laser beam) is located in a predetermined segment of the field of view. . . ". The device must be physically repositioned each time a different spot of tissue is to be treated. In a blood vessel, treatment of a lesion would be limited to one spot at a time. The difficulty of maneuvering the long flexible catheter to a new spot for each small piece of tissue removed, and the likely damage to the delicate vessel wall from repeated and prolonged manipulation of the device would make its use impractical in such a situation. Finally, since the control of the laser power is connected to the position of the hand operated attenuating filter, such control is essentially manual, and is therefore orders of magnitude slower than an electronic control system. It is inadequate for use in a blood vessel where laser radiation can perforate the wall in less than a second. For all these reasons, the Hett instrument is one which is not intended for an inadequate for, use in the vascular system.
Hussein, et al, in U S. Pat. No. 4,445,892, describes a vascular fiber-optic catheter with two inflatable balloons which can seal off a segment of a blood vessel, allowing it to be purged. Blood flow is maintained past the distal end. A cylindrical window allows viewing and laser irradiation through the side of the device. The balloons displace the blood and protect the operating portion of the instrument.
A significant lumen in a vessel must alreadY exist to allow insertion of the balloon distal to the lesion, so the instrument could be used as described. In the cases where the lumen is severely stenosed or restricted, or totally occluded, forcable insertion of the distal balloon may fail or cause serious mechanical injury to the diseased vessel. This instrument is least useful in the situation where the need is greatest. Also as the therapeutic laser radiation is angled to the side to avoid hitting the distal balloon, perforation of the artery wall is more likely than if it were aimed forward. Also, the tube holding the distal balloon restricts the field of view. As with Bass and Hett, the device relies on visualization and the diagnostic information is limited as described. Electronic feedback control of the laser power is not included.
The application of laser catheters have been documented in the literature [D. S. J. Choy, S. H. Sterzer, H. Z. Rotterdam, N. Sharrock and I. P. Kaminow, "Transluminal Laser Catheter Angioplasty", Am. J. Cardiol. 50, 1206-08 (1982); D. S. J. Choy, S. H. Stertzer, H. Z. Rotterdam and M. S. Bruno, "Laser Coronary Artery Angioplasty: Experience with Nine Cadaver Hearts", Am. J. Cardiol. 50, 1209-11 (1982); G. S. Abela, S. Normann, D. Cohen, R. L. Feldman, E. A. Geiser and C. R. Conti, "Effects of Carbon Dioxide, Nd-YAG, and Argon Laser Radiation on Coronary Atheromatous Plaques", Am. J. Cardiol. 50, 1199-1205 (1982); G. Lee, R. M. Ikeda, R. M. Dyer,. H. Hussein, P. Dietrich and D. T. Mason, "Feasibility of Intravascular Laser Irradiation for In Vivo Visualization and Therapy of Cardiocirculatory Diseases", Am. Heart J. 103, 1076-77 (1982); R. Ginsburg, D. S. Kim, D. Guthaner, J. Toth and R. S. Mitchell, "Salvage of an Ischemic Limb by Laser Angioplasty; Description of a New Technique", Clin. Cardiol. 7, 54-58 (1984); and E. Armelin, R. Macruz, M. P. Ribeiro, J. M. G. Brum, M. G. C. Madrigano, P. R. Camargo, J. Mnitentag, P. Pileggi and G. Verginelli, "Application of a Laser Beam in the Vessel Wall Without Interruption of Blood Flow:", Circulation 66 (abstract), II-136 (1982).
In all of these studies the optical fiber conducting the laser light is placed in the artery in an unprotected manner, in direct contact with the blood. Reports in the literature enumerate severe drawbacks in the efficacy and safety of this simple approach. At the tip of the fiber the reaction of the emitted light with the intravascular target is violent. A "crackling" sound during the irradiation process, similar to that of bacon cooking, has been described. The corrosive environment of the blood vessel readily damages the delicate tip of the optical fiber. The light (particularly blue-green argon laser radiation, which is most commonly used) is strongly absorbed by any blood intervening between the tip of the fiber and the tissue target, with the reaction forming debris and gas. There is evidence that red blood cells are damaged, predisposing to the formation of platelet aggregates. In addition to the resultant problem of thrombosis, vascular perforation is a major complication. The latter occurs because of poor control of the laser radiation. Further, even if perforation does not occur acutely, the arterial wall may still be damaged, with the resultant potential for long term aneurysm formation.
Modifications to reduce these complications have been proposed. One approach has been to cover the bare fiber with an absorbing metal tip which is heated by the laser light, forming a hot probe. [T. A. Sanborn, D. P. Faxon, C. C. Haudenschild and T. J. Ryan, "Laser Radiation of Atherosclerotic Lesions: Decreased Incidence of Vessel Perforation with Optic Laser Heated Metallic Tip," J. Am. Coll. Cardiol. (abstract) 3, 490 (1984); G. Lee, R. M. Ikeda, M. C. Chan, J. Dukich, M. H. Lee, J. H. Theis, W. J. Bommer, R. L. Reis, E. Hanna and D. T. Mason, "Dissolution of Human Atherosclerotic Disease by Fiberoptic Laser-Heated Metal Cautery Cap", Am. Heart J. 107, 777-78 (1984)]. This approach is unsatisfactory for several reasons: (i) there is thermal damage to surrounding tissue; (ii) only fatty plaques readily melt away; (iii) the more advanced fiberous and calcified plaques form char and debris; and (iv) the hot tip tends to adhere to the tissue, so when it is removed, the tissue is ruptured.
Despite the scope of the above efforts, a need still exists for accurate control of high power radiation delivered through optical fibers if percutaneous intravascular laser treatment is to reach its full potential.